Traditional transponders used in air traffic radar beacon systems (ATCRBS) for air traffic control (ATC) generally operate such that an encoder may be connected to and in communication with a transponder typically mounted in an instrument panel or avionics rack of an aircraft. The transponder may transmit various codes, including Mode A (reply) codes and Mode C (altitude) codes, but, there is not a mechanism to delineate between these codes (i.e., whether a Mode A code is indeed a Mode A code and not a Mode C code).
Several problems arise from the nature of the design of older-style transponders. Transponders generally transmit information about altitude, and a reply code, assigned by ATC, for use by the ATCRBS. The encoder is generally external to the transponder, and most encoders utilize a parallel wiring mechanism to transfer a Gillham encoded altitude to the transponder. However, if one or more of the parallel wires breaks, or the transponder inputs connected to these parallel wires fail, then the transponder (and ultimately the pilot) may be unable to ascertain whether the altitude information being transmitted to ATC is valid. Older-style transponders also generally use all mechanical means for entry of a reply code, and there is no feedback to the pilot as to the reply code that may be transmitted. For example, if the indicator is integral to the mechanical switch, the switch can fail such that the mechanically connected indicator shows the change, but the transponder circuitry would not detect the change of the failed mechanical switch contacts. Accordingly, the ATC system may be burdened when these older-style transponders fail and transmit false altitude and/or reply codes. Modern transponders generally address these problems by being able to capture altitude and reply codes and show them to the operator or by changing the transponder design; however, the majority of aircraft in operation today utilize these older-style transponders.